The invention relates to an apparatus for providing an electrical indication of low liquid level. Although the invention is particularly suited for liquid reservoirs in automobiles, such as those for hydraulic fluids, lubricating fluids, washer fluids, and cooling fluids, the invention may be used for detecting low liquid levels in any liquid receptacle.
Known low level indicators include a temperature dependent resistive element, such as a thin wire, which is adapted for submersion within a liquid reservoir. The element is electrically heated by either passing a current therethrough or thermally coupling the element to a heating device. Variations in the electrical resistance of the heated element are then detected employing external circuitry. Since thermal transfer to a liquid is greater than thermal transfer to air, an increase in measured resistance provides an indication that the liquid level has receded sufficiently to expose at least a portion of the element to air.
A problem with these approaches is that corrections are required for variations in the ambient temperature of the reservoir and variations in the power supply. Variations in ambient temperature may be particularly severe in an automobile wherein a liquid reservoir may range in temperature from -20.degree. F. to over 200.degree. F. Prior approaches to solving this problem include: the use of sample and hold circuitry (U.S. Pat. No. 4,525,850); differentiating circuitry (U.S. Pat. No. 4,283,719); updating reference circuitry (SAE 800127, entitled "Solid-State Automotive Liquid Level Sensing System", by R. W. Tarpley et al); and, ambient temperature measurements by a separate temperature dependent element and associated compensating circuitry (U.S. Pat. No. 4,457,169). All of these approaches require the use of complicated external circuitry coupled to the sensor.
It is also known to use silicon substrates, and associated fabricating techniques, to provide a liquid level sensing apparatus having greater durability and lower cost than thin wire sensors. However, the response time of the thin wire sensors has been difficult to duplicate. Further, as in the case of hot wire sensors, these approaches also require external circuitry and associated interconnection to compensate for changes in ambient temperature and fluctuations in the power supply.
The following are examples of different sensors which are produced on silicon substrates. U.S. Pat. No. 4,129,848, discloses a silicon substrate with a layer of silicon dioxide and a temperature dependent film of nickel or platinum deposited thereon. In one embodiment, a portion of the silicon substrate is etched away, exposing the layers of silicon dioxide and temperature dependent film. U.S. Pat. No. 4,594,889 discloses a mass airflow sensor having an elongated silicon member positioned between openings through the silicon substrate and a metal deposited on the member. UK patent application No. 2,138,566A discloses a mass flow sensor having multiple resistive elements deposited on a silicon substrate between gaps in the silicon substrate. EPO Pat. No. 0019135 discloses a mass flow sensor having multiple resistive elements deposited on a silicon substrate in a bridge configuration. Another device is discussed in IEEE publication CH2127-9/85/0000-0358, by R. G. Johnson et al, entitled "Design and Packaging of a Highly Sensitive Microtransducer For Air Flow and Differential Pressure Sensing Applications." The publication shows a gas flow sensor fabricated from a silicon chip having a temperature sensitive resistor mounted on a dielectric film suspended over an etch pit in the chip.
There still remains a need for a low liquid level sensor having a fast response time which is economically manufactured by conventional microelectronic batch processing techniques known in the microelectronics industry. In addition, a need remains for a sensor which is produced on a single chip and does not require corrections for changes in ambient temperature conditions and power fluctuations. Further, it would be advantageous to provide a single sensor suitable for use with a variety of different liquids, each having a different degree of corrosivity, thermal transfer, and/or electrical conductivity. These are some of the problems the invention overcomes.